Generation of a strong, highly uniform magnetic field is required for nuclear magnetic resonance imaging (MRI). In MRI, there is the ability to form images of biological tissue in vivo. With this ability, MRI of humans for medical diagnostic purposes can be utilized for the early detection of tumors and the like. The MRI phenomenon is a result of the magnetic properties of atomic nuclei and the ability to observe nuclear spin distributions in the presence of a magnetic field. The magnetic field needed for MRI can be generated by electromagnets or permanent magnets.
Different types of magnet systems have been proposed and utilized in an attempt to obtain a highly uniform field, and still realize a reasonable initial cost, simplify site preparation, control cost of operation, and minimize maintenance requirements. Specifically, the types of magnets used for this purpose include superconducting magnets, resistive magnets, and permanent magnets.
Of help in evaluating these magnets are the observations of W. Oldendorf in his article "Nuclear Magnetic Resonance and Correlative Imaging Modalities," published in the Society of Nuclear Medicine pp. 45-54, 1983, wherein he compares the advantages and disadvantages of superconducting, resistive and permanent magnets for human MRI. In this publication, Oldendorf notes that superconducting magnets have the advantage of a strong field with high uniformity. He also notes they are, however, expensive to buy and maintain, and that they typically have a large fringe field. Furthermore, they require extensive site preparation. Moreover, they generate only a longitudinal field and require a vacuum and the consequent problems associated with the handling of liquified gases. In comparison, resistive magnets can also have good uniformity and generate a transverse field. In addition, they are relatively inexpensive and require no vacuum or handling of gases. Unfortunately, the strength of resistive magnets is limited, and they further require an elaborate and costly power and water supply. In addition, they require extensive site preparation due to their large weight. Lastly, in comparison, permanent magnets are inexpensive, have minimum site preparation requirements, generate minimal unwanted fringe field, and have no power supply, liquified gas handling or vacuum requirements. Their disadvantages are, however, that permanent magnets have limited field strength, temporal instabilities, are very heavy, and have a field whose uniformity does not meet industry requirements to date. Nevertheless, permanent magnets appear to be a newly developing, cost effective solution for MRI.
As one solution to the problem of creating a uniform and homogeneous flux field for MRI, a general design of a permanent magnet MRI structure has been proposed by Oldendorf which utilizes an external frame of iron supporting two opposed permanent magnets.
To solve the uniformity problem when using permanent magnet systems for MRI tomography, other authors have proposed various types of systems. Unfortunately, most such systems, in order to obtain the field uniformity desired, require an enclosed magnetic field. An enclosed field, however, is not practical as it limits the ease with which the patient can be positioned in the magnetic field. See, for example, "Permanent Magnetic Systems for MRI Tomography" by H. Zijlstra, Philips Journal of Research, Volume 40, No. 5, 1985, pp. 259-288. In this article, it is noted that the standard requirement for a magnet system is that it be a full-body magnet with an access diameter of approximately one (1) meter, and a magnetic field uniformity of no more than one hundred (100) parts per million (ppm) within a sphere of one half (0.5) meters diameter. As a practical matter, most MRI devices barely exceed these requirements. It is desirable to attain field uniformity of up to forty (40) parts per million over a spherical volume having a diameter of approximately thirty (30) centimeters.
Various attempts to solve some of the problems involving permanent magnets have been made which include substantially parallel flat plates that support opposed permanent magnets and which are coupled together by a plurality of rod-like yoke portions. Such a device is disclosed in U.S. Pat. No. 4,672,346 to Miyamoto et al. It also includes complicated adjusting mechanisms for moving magnetic materials about with respect to the plates. It further contemplates use with permanent magnets showing a large maximum energy product, on the order of two hundred forty (240) kilojoules per cubic meter, to attain a uniformity of two hundred (200) ppm inside a spherical volume of three tenths (0.3) meters in diameter. It also discloses that addition of a field compensating (heated) shim may improve the uniformity further.
Various attempts have been made to obtain uniform flux fields by varying the pole face topography to attain homogeneity of the flux. One known method which accomplishes some degree of control involves providing a peripheral annular shaped rim which is positioned on the pole face of the magnet. With this method the positions of the central pole faces and rims can be adjusted independently in attempts to e establish a uniform flux field. However, limitations to uniformity have been observed as a result of even slight asymmetries in the geometry of the device. Furthermore, imperfections in the iron or other material being used for the magnet can affect the uniformity in magnetization. See for example the article entitled "Field Homogeneity and Pole Distribution," by J. D. Bjorken and F. Bitter, published in the Review of Scientific Instruments, Volume 27, No. 12, December 1956, pp. 1005-1008.
Regardless of the type of magnet used to generate the flux field for MRI purposes, the field needs to have several desirable characteristics. Most importantly, the flux field needs to be uniform and homogeneous in the space where MRI is to be accomplished (i.e. all lines of flux need to be substantially parallel to each other). Also, for overall efficiency, the generation of the flux field should be accomplished as efficaciously as possible. It is necessary further that the apparatus can be easily and conveniently adjustable to attain a highly uniform field once it is at the installation site, e.g. the medical facility. It is desirable to adjust an MRI apparatus in the factory to within two hundred to three hundred (200-300) ppm and then be able to finely adjust the field uniformity to greater uniformity, e.g. thirty to forty (30-40) ppm, without requiring extensive operator training or complicated adjustment apparatus and methods. Such fine adjustments of the field uniformity include minimizing flux field errors to provide greater resolution in magnetic resonance imaging. Attaining such fine tuned condition in the factory would be impractical and inefficient since shipping, handling and environmental site conditions introduce field errors which can only be corrected after installation of the MRI system at the site. However, such method and apparatus to adjust the uniformity to the required level must be simple and convenient to utilize for practical acceptance of such devices.
The present invention also further recognizes that flux field uniformity can be improved by incorporating a method and apparatus superposed adjacent the magnet's pole face for adjustment to establish the required effective uniform field. The field can be defined in three dimensions, x, y, and z, by the known Legendre polynomial expansion, which has coefficients which describe the x, y, and z gradients of the magnetic field. Flux uniformity can be achieved by minimizing lower order error perturbations of the Legendre expansion, or minimizing Legendre coefficients, which account for small variations, by making adjustments at the site. Minimizing higher order perturbations, which account for larger variations, can be more readily accomplished at the factory.
Accordingly, it is an object of the present invention to provide a method and apparatus which is easy and convenient to use to achieve a highly uniform magnetic field suitable for use in MRI. It is another object of the present invention to provide a magnetic field control apparatus which is relatively inexpensive to manufacture, install, operate and maintain while achieving reliable results in operation. It is another object of the present invention to provide a system and apparatus for adjustably fine tuning the uniformity and resolution capability of the magnetic field at the site. Other objects of the present invention will become apparent in the full description of the invention taken in conjunction with the drawings set forth below.